Chlorine-containing resins, and particularly vinyl chloride resins, are known to have a drawback in stability against light and heat and tend to cause decomposition primarily due to dehydrohalogenation upon thermal molding or product use. Various attempts have been made to improve the stability of vinyl chloride resins by mixing various types of stabilizers, such as metal salts of organic acids, organic tin compounds, organic phosphite compounds, epoxy compounds, β-diketone compounds, antioxidants, and UV absorbers.
In recent years, the use of harmful heavy metals such as lead and cadmium, which are toxic from the standpoint of environmental protection, has come to be avoided and restricted. This has aroused a demand for replacement of such substances with nontoxic or low-toxic composite stabilizers, such as barium-zinc-based stabilizers, magnesium-zinc-based stabilizers, calcium-zinc-based stabilizers, and calcium-magnesium-zinc-based stabilizers.
However, in vinyl chloride resin compositions that include zinc-containing composite stabilizers, zinc chloride is produced as thermal decomposition begins, and the zinc chloride acts as a catalyst and advances degradation of the vinyl chloride resin. A method that is known for preventing this action is to use, in combination, a polyol compound to deactivate the zinc chloride through chelation. Examples of such polyol compounds include pentaerythritol and pentaerythritol condensates such as dipentaerythritol and tripentaerythritol.
Pentaerythritol, however, causes sublimation, which may give rise to problems such as contamination of processing devices and damage to the surfaces of products made of the vinyl chloride resin composition. Accordingly, it is necessary to keep the content of pentaerythritol at least equal to or below 10% by mass with respect to the total amount of the mixture of the above-mentioned polyol compounds, and it is preferable to keep the content of pentaerythritol equal to or below 1% by mass with respect to the vinyl chloride resin composition.
Pentaerythritol condensates such as dipentaerythritol and tripentaerythritol are compounds that are good in providing thermal stability to vinyl chloride resin. These compounds, however, have melting points higher than the molding/processing temperature of vinyl chloride resin and also have poor compatibility with vinyl chloride resin, resulting in poor dispersion within the resin. This leads to plate-out and results in problems such as insufficient effect of providing thermal stability and deterioration in transparency of molded products. Accordingly, various attempts have been made heretofore in order to solve these problems caused by poor dispersion.
An example of a method for resolving the above-mentioned poor dispersion is given in Patent Document 1, which discloses fine-grinding of pentaerythritol.
Patent Document 2 discloses a method of melting a mixture of ditrimethylol propane and dipentaerythritol to improve the dispersibility of dipentaerythritol.
Patent Document 3 discloses a partially-esterified compound produced by reacting pentaerythritol and at least one compound selected from a group consisting of monocarboxylic acids, polycarboxylic acids, esters thereof, acid halides thereof, and acid anhydrides thereof. Patent Document 4 discloses ester compounds such as a partially-esterified compound produced by esterification of pentaerythritol and a linear saturated fatty acid.
Patent Document 5 discloses a method of melting and mixing dipentaerythritol and ester compounds of pentaerythritols.
Patent Document 6 discloses a method of producing a stabilizer for vinyl chloride resin through dehydrative condensation reaction by esterification of pentaerythritol and a dibasic acid (or an acid anhydride thereof).
Patent Document 1: JP-A-10-7859
Patent Document 2: JP-A-9-268286
Patent Document 3: JP-A-56-57844
Patent Document 4: JP-A-7-97495
Patent Document 5: JP-A-2003-336064
Patent Document 6: JP-A-8-143704
Excellent properties/characteristics are required of current-day vinyl chloride resins, and there is a demand for further improvement in heat resistance (thermal stability) and in properties such as transparency. Accordingly, the current situation is that vinyl chloride resins containing conventionally-proposed stabilizers are no longer sufficient.
For example, the method disclosed in Patent Document 1 makes it possible to resolve the above-mentioned problem of poor dispersion by pulverizing the pentaerythritol compound to a particle diameter of 35 μm or less. This pulverizing process, however, requires a large amount of energy and thus gives rise to an increase in cost.
As for the method disclosed in Patent Document 2, ditrimethylol propane contained in the mixture of ditrimethylol propane and dipentaerythritol has no effect as a stabilizer. Accordingly, in the mixture of ditrimethylol propane and dipentaerythritol, the number of dipentaerythritol (the hydroxy groups), which is the primary effective component, is reduced, and it is thus necessary to use a large amount of the mixture to attain the desired properties/characteristics. Using a large amount of the mixture, however, gives rise to deterioration in the physical properties of the molded products and also causes plate-out on the surfaces thereof.
The partially-esterified compound produced by reacting pentaerythritol and at least one compound selected from a group consisting of monocarboxylic acids, polycarboxylic acids, esters thereof, acid halides thereof, and acid anhydrides thereof disclosed in Patent Document 3 and the ester compounds such as the partially-esterified compound produced by esterification of pentaerythritol and a linear saturated fatty acid disclosed in Patent Document 4 are inferior in heat resistance compared to pentaerythritol itself, and the number of hydroxy groups therein, which are the primary effective components, is reduced, thus being far from satisfying as a stabilizer.
The mixture produced by melting and mixing dipentaerythritol and ester compounds of pentaerythritols disclosed in Patent Document 5 is reduced in the number of hydroxy groups, which are the effective components as a stabilizer. Further, the molten mixture is taken out from a mold in the form of a large clump, and there is a need for an additional process of pulverizing the clump to a size/shape susceptible of use.
The condensate produced by esterification of pentaerythritol and a dibasic acid (or an acid anhydride thereof) disclosed in Patent Document 6 contains etherified compounds of pentaerythritol and is thus reduced in the number of hydroxy groups, which are the effective components as a stabilizer, thus being far from satisfying.
As described above, the various attempts made heretofore have not been successful in providing a satisfying stabilizer for chlorine-containing resins and a chlorine-containing resin composition using such stabilizer.